Starting with Exchange 2007, neither the Messaging API (MAPI) client libraries nor CDO 1.2.3 are provided as a part of the base product installation. As a result, there is functionality missing that many applications depend on. Microsoft Exchange MAPI and CDO 1.2.3 provide access to these APIs

The Messaging API is a COM-like API that provides access to the contents of messaging stores. CDO 1.2.3 (Collaboration Data Objects, version 1.2.3) is a package providing access to Outlook-compatible objects through a COM-based API. Using either CDO or MAPI, a program can connect to a MAPI store, and then perform operations against that store. Starting with Exchange 2007, Microsoft will distribute the MAPI client libraries and CDO 1.2.3 as a Web download.